A photovoltaic power generation system includes a solar cell string in which a plurality of solar cell modules are connected in series. Each solar cell module includes a plurality of solar cells connected in series, and a bypass diode (BPD) for bypassing the current path of the solar cells is connected in parallel to each solar cell module.
Defects may arise in such a solar cell string, such as malfunctions in solar cell modules, disconnects in cables connecting solar cell modules to each other, opening of the BPD (due to disconnects or the like), or ground faults. Thus in a photovoltaic power generation system, it is necessary to appropriately check for defects in the solar cell string to ensure a stable supply of power and to prevent accidents. The techniques disclosed in JP 2016-050783A, JP 2016-093039A, and JP 2014-011427A are known as techniques for checking for defects in a solar cell string.
A ground fault detection apparatus disclosed in JP 2016-050783A opens a negative terminal of a solar cell string and grounds a positive terminal via a detection resistance, and a voltage arising over both ends of the detection resistance at this time is detected as a first voltage. Likewise, the positive terminal of the solar cell string is opened and the negative terminal is grounded via a detection resistance, and a voltage arising over both ends of the detection resistance at this time is detected as a second voltage. Furthermore, an inter-terminal voltage between the positive terminal and the negative terminal of the solar cell string is detected.
Next, the ground fault detection apparatus finds a ground fault resistance value (insulation resistance value) of the solar cell string on the basis of the first voltage, the second voltage, the inter-terminal voltage, and the resistance value of the detection resistance. Whether or not there is a ground fault can be detected from this ground fault resistance value. Furthermore, the position of a ground fault can be detected from a ratio between the first voltage and the second voltage.
An inspection apparatus disclosed in JP 2016-093039A sequentially applies an AC inspection signal to the positive terminal and the negative terminal of a solar cell string and measures an impedance in both cases. Whether or not there is a cable disconnect or whether or not there is a defect in the solar cell module can be detected, and furthermore, the position of the cable disconnect or the position of the defective solar cell module can be detected, from the ratio between the impedance obtained when the inspection signal is applied to the positive terminal and the impedance obtained when the inspection signal is applied to the negative terminal.
A defect detection apparatus disclosed in JP 2014-011427A sends current at a prescribed current value toward the positive terminal of a solar cell string from the negative terminal of the solar cell string, measures a potential difference between the negative terminal and the positive terminal of the solar cell string, and determines whether or not a bypass diode is defective on the basis of the measured potential difference.
JP 2016-050783A (published Apr. 11, 2016), JP 2016-093039A (published May 23, 2016), and JP 2014-011427A (published Jan. 20, 2014) are examples of the background art.